This invention relates generally to implantable cardiac pacemakers and, more particularly, to cardiac pacemakers having oxygen sensors of the kind that determine oxygen content by measuring the blood's reflectivity.
The cardiac pacemaker is perhaps one of the best known electronic marvels of modern medicine, and the implantation of a pacemaker in a patient has become almost a routine operation. The pacemaker pulses the patient's heart muscle continuously over an extended period of time, or in the case of demand pacemakers, monitors the heart muscle's natural operation and provides stimulating pulses only when the heart muscle skips a beat. Pacemakers allow patients with heart problems that otherwise would have been fatal or incapacitating to resume relatively normal lives.
The modern pacemaker is a highly complex device, capable of event sensing, two-way telemetry, and sensing and pacing in either or both of the atrium and the ventricle of the heart muscle. Such pacemakers may be finely tuned by the physician subsequent to implantation, and the parameters adjusted to provide optimum pacing performance.
Despite the sophistication of such pacemakers, a major difference remains between the healthy heart muscle and a paced heart muscle--namely, the response to activity or exercise. Variations in the cardiac stroke volume and systemic vascular resistance occur in the cardiovascular system due to physiological stresses such as exercise, temperature changes, postural changes, emotion, hypoglycemia, Valsalva maneuvers, etc.
To maintain adequate perfusion pressure and cardiac output under these stresses, it is necessary to adjust heart rate. The healthy heart muscle might beat at 60 or fewer beats per minute during repose or sleep, and at 120 or more beats per minute during strenuous exercise, for example. The heart muscle paced by a pacemaker that is non-rate responsive will typically beat at a constant rate of approximately 70 beats per minute.
It will be appreciated that the constantly-paced heart muscle will supply more blood than is needed during sleep, and might even prevent the patient from sleeping restfully. Even more seriously, patients paced at 70 beats per minute experience substantial difficulty in engaging in strenuous activity. Even a moderate level of activity such as walking will cause difficulty in some patients. It is apparent that a demand pacemaker whose rate varies in response to physiological need represents a highly desirable device that will enable patients requiring pacemakers to lead normal, active lives.
Physiologically-responsive cardiac pacing must optimize cardiac rate to the level of metabolic need in the absence of a normal variable cardiac rate. The simplest solution to this problem is atrial tracking pacing, where the patient has a full or partial AV block and a dual chamber pacemaker pulses the ventricle in response to normal cardiac activity sensed in the atrium. However, this technique is not possible in many patients with sinus bradycardia or atrial fibrillation, and so rate-responsive pacing is necessary to mimic the normal variable cardiac rate.
A variety of physiologically-responsive pacing systems have been proposed, which utilize a variety of physiological parameters as the basis for varying cardiac rate. These parameters include blood temperature, various sensed timing signals from the heart muscle, pressure measured within the heart muscle, respiratory rate, nervous system activity, physical activity, and blood chemistry.
Systems responsive to various blood chemistry parameters, such as blood oxygen saturation, are particularly worthwhile and effective. One such oxygen sensor is disclosed in U.S. Pat. No. 5,040,538, issued to Said Mortazavi and entitled "Pulsed Light Blood Oxygen Content Sensor System and Method of Using Same," which is incorporated herein by reference. The disclosed sensor includes an optical detector for measuring the mixed venous oxygen content, typically in the heart muscle's right ventricle. A diminution in the mixed venous oxygen content is used to produce a higher paced cardiac rate. The speed of this system is comparable to the time constant of the body, thereby enhancing its effectiveness.
The oxygen sensor disclosed in the prior patent includes a light-emitting diode or LED, positioned within the heart muscle's right ventricle and arranged such that any light it emits is directed at the blood within the ventricle, which reflects the light to an adjacent phototransistor. The amount of light so reflected is inversely related to the blood's oxygen content. The phototransistor is part of a circuit that is connected in parallel with the LED. When an electrical current pulse is supplied to the LED, the circuit begins integrating the resulting phototransistor current. When the integrated voltage reaches a predetermined threshold, the circuit latches and diverts the current pulse from the LED, to terminate its generation of light. The time delay from initiation of the measuring current pulse to latching of the circuit is inversely related to the blood's oxygen content level.
Two techniques for pacing the heart muscle are in common use. One technique, referred to as bipolar pacing, requires the implantation of two conductors. In this technique, the electrode end of one conductor is implanted in contact with the heart muscle while a second conductor is implanted with its electrode end in contact with blood in the heart muscle. Current pulses between these two conductors stimulate the heart muscle. A second technique, referred to as unipolar pacing, requires only a single conductor with its electrode end implanted in contact with the heart muscle. Instead of a second conductor, this technique uses the pacemaker's case, which is electrically connected to the pacemaker's circuitry, to provide an electrical connection to body tissue in the chest. This unipolar approach is subject to certain drawbacks. In this approach, the pacing pulse can stimulate not only the heart muscle, but also muscles in the chest that are located between the implantation site of the pacemaker and the heart muscle. Additionally, when the pacemaker needs to sense heart muscle activity, it also can sense activity of other muscles in the chest. Although not insurmountable, these are undesirable consequences.
Ideally from the standpoint of electrical simplicity, the interface to the oxygen sensor would consist of two conductors that are physically distinct from the two conductors used in bipolar pacing of the heart muscle. However, these four conductors would have to pass together as a single pacing lead through the vein, through the heart valve, and into the right ventricle of the heart muscle. As the number of conductors increases, the thickness and the stiffness of the pacing lead increases proportionally. This increases the difficulty of implanting the pacing lead into the heart muscle as well as the difficulty of the heart valve closing on the thicker pacing lead. Additionally, the pacing lead must connect to the pacemaker through a connector. As is well known in the art, the reliability of an electrical circuit generally will decrease with an increase in the number of connections. It is clearly imperative that a life-supporting device, such as a pacemaker, have as high a degree of reliability as possible. Thus, while it is highly desirable to add the feature of oxygen sensing to a pacemaker system, it would be even more advantageous if this feature could be added without increasing the number of conductors in the pacing lead.
In U.S. Pat. No. 5,040,538, this problem is addressed by committing an additional conductor to the oxygen sensor and by referencing the signals on this conductor to an electrical connection in common with the pacing signal. In a bipolar configuration, this results in a three-conductor implementation, while, in a unipolar configuration, this results in a two-conductor implementation. While accomplishing a goal of a two-conductor pacemaker with oxygen sensing, this implementation relies upon a unipolar approach with its previously-described drawbacks.
Thus, a need exists to provide the performance advantages of physiologically-responsive pacing by using an oxygen sensor but without increasing the number of conductors already required for bipolar or unipolar pacing. Thus, for a bipolar system an interconnection apparatus is needed that can additionally provide oxygen sensing with only two conductors, while for a unipolar system this requirement should be met with only a single conductor. The present invention satisfies these needs.